The invention relates to a method for selecting a way to perform a handover in a cellular radio system employing code division multiple access, and comprising in each cell at least one base station transmitting a pilot signal, and a number of subscriber terminal equipments that may be simultaneously connected to one or more base stations, said terminal equipments measuring pilot signals received, and being capable of performing handovers of at least two types.
The present invention is suited to be used in radio systems utilizing code division multiple access, and in particular in so-called WLL (Wireless Local Loop) systems. Code Division Multiple Access, CDMA is a multiple access method, which is based on the spread spectrum technique, and which has been applied recently in cellular communication systems, in addition to the prior FDMA and TDMA methods. CDMA has several advantages over the prior methods, such as spectral efficiency and the simplicity of frequency planning.
In CDMA, the narrow-band data signal of the user is multiplied to a relatively wide band by means of a spreading code having a remarkably broader band than the data signal. Bandwidths used in known test systems are e.g. 1.25 MHz, 10 MHz and 25 MHz. In connection with the multiplication, the data signal spreads onto the whole of the band used. All users transmit simultaneously by using the same frequency band. An individual spreading code is used on each connection between the base station and the mobile station, and the signals of different users may be distinguished from each other in the receivers on the basis of the spreading code of each user.
Adapted filters in the receivers are synchronized with the desired signal, which is identified on the basis of the spreading code. The data signal is returned in the receiver onto the original band by multiplying it by the same spreading code as in connection with the transmission. The signals which have been multiplied by some other spreading code neither correlate nor return to the narrow band in an ideal case. They thus appear as noise from the point of view of the desired signal. It is endeavoured to select the spreading codes of the system so that they are not mutually correlated, in other words, they are orthogonal. In practice, the spreading codes are not non-correlated, and the signals of other users complicate the detection of the desired signal by distorting the received signal. This interference caused by the users for each other is termed as multiple access interference.
In several CDMA systems in accordance with the prior art, a so-called pilot signal is used, which is transmitted by each base station and utilized in identification of base stations, power measurement, and for enabling coherent reception in a mobile station. The pilot signal is a data unmodulated spreading coded signal, and it is transmitted to the coverage area of the base station in the same way as the actual traffic channels. The coverage area of the pilot signal in fact determines the size of the cell of the base station, since the terminal equipments utilize it in call establishment.
The base stations continuously transmit the pilot signal, and the spreading codes used in the transmission of the pilot signal of different base stations differ from each other, so that a terminal equipment may identify the base stations on the basis of the pilot transmitted by them.
Systems in which a radio telephone network is used for replacing a conventional fixed telephone network are termed as WLL systems. In a WLL system, subscriber terminal equipments are fixed on the premises of the user, like a conventional telephone. In all data transmission methods utilizing a radio channel, interference occurs on the radio path, such as multipath fading of the signal, which is due to summing of such a signal in the receiver that has propagated several different ways. Since in a WLL system both the transmitter and the receiver are stationary, multipath fading is caused by the movements of the objects located in the surroundings. Thus, radio technically speaking, fading is very slow. A fading period usually varies from one second to a few seconds. Moreover, the fading depths may be so remarkable that they have an effect on the quality of the connection, in particular if the subscriber terminal equipment is located in the peripheral area of two cells.
A terminal equipment located in a peripheral area may meet with a situation in which a signal received from its own base station is fading just when a signal arriving from an adjacent base station is several decibels stronger than the desired signal. The signal-to-interference ratio of the signal received from its own base station, i.e. the quality of the connection may then decrease significantly. It is thus necessary to use a handover method of some kind between different base stations in the peripheral areas of a cell, which also takes multipath fading into account.
Known cellular radio systems, such as GSM, employ so-called hard handover, in which the terminal equipment communicates only with one base station at a time. If the quality of the connection weakens, a handover is performed to a new base station, the signal received from which is stronger.
CDMA cellular radio systems usually employ so-called soft handover. In soft handover, the terminal equipment may simultaneously communicate with more than one base stations during the handover. The base stations with which the terminal equipment may communicate are referred to as an active group. All the base stations with which the terminal equipment communicates transmit the same signal to the terminal equipment, which may select from said signals the strongest signal components, and advantageously combine them. The signal-to-interference ratio of the received signal may thus be maximized.
A parallel international application xe2x80x9cHandover method and a cellular radio systemxe2x80x9d PCT/FI95/00467 discloses a method in which the terminal equipment directly controls the transmission power of the base stations during a soft handover. By means of the method, it is possible to switch only the necessary base stations of the active group to transmit the signal to the terminal equipment.
Known methods, however, are attended by numerous drawbacks. If a hard handover is employed in a WLL system, taking multipath fading into account, it may result in a so-called ping-pong phenomenon, in which the connection is continuously switched between two base stations. This causes relatively much signalling on the radio path.
The drawback of soft handover in the WLL system is that extra interference power may unnecessarily be sent onto the radio path. Since the fading process of the signals sent from different base stations is not correlated, the signal received from one base station may be at its strongest simultaneously when the signal received from another base station is fading. If the fading is several decibels, the signal received from the latter base station is hardly of avail in the terminal equipment. Said signal, however, causes interference in the coverage area of said base station, and the capacity of the cell thus decreases. In conventional cellular radio systems, this problem is not relevant, as the fading processes are much faster. The disclosed situation may, however, last several seconds in the WLL system.
The method disclosed in a parallel international application xe2x80x9cHandover method and a cellular radio systemxe2x80x9d PCT/FI95/00467 is well suited for a WLL environment if the fading dynamics is relatively high, and the need for handover does not arise too frequently. It must be taken into account in the WLL system, however, that different directions of transmission are processes independent of each other. If the power control commands in the uplink direction are sent to the terminal equipment on the basis of the best connection in the uplink direction, it may be the case that the base station the downlink signal received from which is the best, does not hear the signal in the uplink direction.
Each of the above mentioned handover methods has its drawbacks that waste capacity and resources if the same method is used in the whole system.
The aim of the present invention is to implement a method in which the handover solution most appropriate for each situation is selected, said solution wasting as little capacity and resources as possible.
This is achieved with a method of the type set forth in the introduction, which is characterized in that the terminal equipment makes, on the basis of the pilot signals received by it, the decision on the handover technique used on the connection between the terminal equipment and the base station each time.
The invention further relates to a cellular radio system employing code division multiple access, and comprising in each cell at least one base station transmitting a pilot signal, and a number of subscriber terminal equipments that may be simultaneously connected to one or more base stations, said equipments comprising means for measuring the pilot signals received by them, and comprising means for performing handovers of at least two types. The cellular radio system of the invention is characterized in that the terminal equipment of the system comprises means for selecting on the basis of the pilot signals received by it the handover technique which is used on the connection between the terminal equipment and the base station each time.
By means of the method of the invention, transmission and processing of extra data connected with conventional methods may be avoided. On the basis of the measurements of the pilot signals, the terminal equipment may find out in advance which handover technique is the most advantageous each time. The terminal equipment measures pilot signals during a time period the length of which is essentially greater than the fading period of the radio channel. In WLL systems, the measuring time period may be as long as a half an hour. By means of the measurements during the connection, the way to perform the handover may also be changed during the connection. In a preferred embodiment of the invention, the terminal equipment calculates the power difference between the weakest and the strongest signal of the pilot signals received by it. In addition, the terminal equipment calculates on the basis of the pilot signals measured by it what is the estimated time distribution of handovers when only hard handovers are performed. On the basis of these parameters, the terminal equipment may conclude which handover technique is the most advantageous each time.